Ion implanters are widely used in semiconductor manufacturing to selectively alter the conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components which include one or more magnets and a plurality of electrodes.
FIG. 1 shows a known ion implanter 100 which comprises an ion source 102, extraction electrodes 104, an analyzing magnet 106, a first deceleration (D1) stage 108, a collimating magnet 110, and a second deceleration (D2) stage 112. The analyzing magnet 106 may select desired ion species and filters out contaminant species. The collimating magnet 110 may make the beam substantially parallel when it impacts the workpiece. The D1 and D2 deceleration stages (also known as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass therethrough. By applying different combinations of voltage potentials to sections of the beam line and to the multiple electrodes, the D1 and D2 deceleration stages can manipulate ion energies and cause the ion beam to implant a target workpiece 114 at a desired energy.
As the semiconductor industry keeps reducing feature sizes of micro-electronic devices, ion beams with lower energies are desirable in order to achieve shallow dopant profiles for forming shallow junctions. Meanwhile, it is also desirable to maintain a high beam current in order to achieve a high production throughput. Such low-energy, high-current ion beams may be difficult to transport within typical ion implanters due to the drop in efficiency of ion transportation. Therefore, the ion beam may be transported initially at high-energy, and then decelerated before a collimating magnet and/or before a workpiece. However, a portion of the ion beam may react with ambient gases and become neutral due to charge exchange before being decelerated. The neutral portion of the ion beam may not be sufficiently decelerated and will remain as energetic fast neutrals. Thus, the workpiece may be implanted with both low-energy ions and energetic fast neutrals to cause a degradation of the ion implantation process in such parameters as absolute dose, dose uniformity, and/or dopant depth profile.
Existing ion monitoring tools often lack the capability of providing detailed real-time composition information of an ion beam. In a typical ion implanting process, for example, the ion beam may be controlled by monitoring an implant dose based on a Faraday cup current. However, a Faraday cup is just a total charge counter which cannot detect neutral particles. Schemes have been used before in which neutral particles were separated from charged particles by using electrostatic fields, and the neutral particles then detected by impinging them onto a plate, and measuring the secondary electrons that are thereby emitted. However, difficulties may be encountered in measuring the small signal level of the neutral particles in the presence of the much larger ion signal, and due to the secondary electron coefficient of the plate changing with time.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with present techniques for sensing neutrals particles and to ensure that predetermined energy contamination requirements are satisfied at all times during implantation processes.